1. Field of the Invention
The present invention relates to an organic electroluminescence (organic EL) device having excellent light-emitting properties.
2. Description of the Related Art
An organic electroluminescent device is a light-emitting device which makes use of the principle that when an electric field is applied, a fluorescent material emits light in response to the charger combination of holes injected from an anode and electrons injected from a cathode.
Since C. W. Tang et al. of Eastman Kodak Company made are port on a low-voltage-driven organic EL device using a double layer structure (C. W. Tang, S. A. Vanslyke, Applied Physics Letters, 51, 913(1987) and the like), studies on an organic EL device and materials have been briskly carried out.
Tang et al. applied tris(8-hydroxyquinolinol aluminum) to an emission layer and a triphenyldiamine derivative to a hole-transporting layer of multi-layered EL device. This stacked structure is accompanied with such advantages as an improvement in the injection efficiency of holes into the emission layer; blocking of electrons injected from a cathode, which increase the efficiency of exciton production from charge recombination; and confine of the excitons into the emission layer. As described above, a double layered structure composed of a hole-injecting and transporting layer and an electron-transporting and emission layer or a triple layered structure composed of a hole-injecting and transporting layer, a emission layer and an electron-injecting and transporting layer is well known as an organic EL device. In order to increase the recombination efficiency of injected holes and electrons, various improvements in the device structure or fabrication process have been introduced to such multi-layered devices.
As a hole-transporting material, triphenylaminederivatives such as 4,4xe2x80x2,4xe2x80x3-tris(3-methylphenylphenylamino)triphenylamine and aromatic diamine derivatives such as N,Nxe2x80x2-diphenyl-N,Nxe2x80x2-bis(3-methylphenyl)-[1,1xe2x80x2-biphenyl]-4,4xe2x80x2-diamine, which are starburst molecules, are well known (e.g., Japanese Unexamined Patent Application, First Publication Nos. 8-20771, 8-40995, 8-40997, 8-53397 and 8-87122).
As an electron-transporting material, oxadiazole derivatives, triazole derivatives and the like are well known.
As a light-emitting material, known are chelate complexes such as tris(8-quinolinolate)aluminum complex, coumarin derivatives, tetraphenylbutadienederivatives, bisstyrylarylene derivatives, oxadiazole derivatives and the like. Since these light-emitting materials can emit various color luminescences in a visible region from blue to red, there increased expectation for industrialization of a full color organic EL device (e.g. Japanese Unexamined Patent Application, First Publication Nos. 8-239655, 7-138561, 3-200889 and the like).
Organic EL device shaving high brightness and high efficiency have been disclosed or reported in recent years. However, in the many cases of organic EL devices having high brightness and high efficiency, the light emitted from materials in not only the emission layer but also the electron transporting layer are observed and that makes hard to get an organic EL device with pure color, especially pure emission.
Accordingly, the object of the present invention is to provide an organic EL device having high luminous efficiency as well as pure color light emission.
For the purpose of suppression light emission from the material in the electron transporting layer, the layer fabricated between the emission layer and the electron transporting layer to prevent injection of holes from emission layer to electron transporting layer and dispersion of excitons formed at the interface is considered to be more effective the greater the film thickness.
However, the inventors of the present invention found that, although emission from the material in the electron transporting layer is not observed in the case of a device comprising a intermediate layer using a material with an ionization potential larger than that of the material used for the emission layer and having thickness of 1 to 20 nm between the emission layer and the electron transporting layer, light emission from the material in the electron transporting layer is not suppressed not only in the case in which the film thickness of the intermediate layer is less than 1 nm, but also in the case the film thickness exceeds 20 nm, thereby leading to completion of the present invention.
In addition, if the materials in the emission layer, electron transporting layer and intermediate layer are typical semiconductors, hole injection from the emission layer into the electron transport layer is suppressed when the relationship of ionization potentials of materials is Ip1 less than Ip2 less than Ip3, wherein Ip1, Ip2 and Ip3 represent the ionization potentials of materials used for the emission layer, electron transporting layer and intermediate layer, respectively, and promoted when their relationship is Ip1 less than Ip3 less than Ip2. In the latter case, this is considered that the light emission from a material in electron transporting layer caused by carrier recombination in the electron transporting layer should increase. In the case of an organic EL in which the film thickness of an intermediate layer fabricated between an emission layer and electron transporting layer is 25 nm, increase of light emission from the material in the electron transporting layer was observed when the relationship is Ip1 less than Ip3 less than Ip2, actually.
However, the inventors of the present invention found that light emission from the electron transporting layer is suppressed in the case the film thickness of an intermediate layer is 1 to 20 nm, relationship of ionization potentials is Ip1 less than Ip3 less than Ip2.
Moreover, the inventors of the present invention found that an above-mentioned organic EL device with the emission layer in contact to anode perform emission with deeper color.
An organic EL device according to the present invention comprises at least an emission layer and an electron transporting layer between an anode and a cathode and has an intermediate layer that has a film thickness of 1 to 20 nm and is made of a material having a larger ionization potential than that of the material used for the emission layer.
In addition, the material that forms the above described intermediate layer may have an ionization potential that is smaller than that of the material used for the electron transporting layer.
In addition, the above emission layer is preferably adjacent to the anode.
The organic EL device according to the present invention has an intermediate layer that has a film thickness of 1 to 20 nm and is made of a material having a larger ionization potential than that of the material used for the emission layer and the device emits light with deep color highly effectively.
In addition, even if the material that forms the above intermediate layer has a smaller ionization potential than that of the material used for the electron transporting layer, the device still emits light with deep color highly effectively.
In addition, if the above emission layer is adjacent to an anode, the device emits light with deeper color.